My long-range objective is to understand the mechanisms that produce smooth eye movement appropriate to stabilize retinal images. The vestibulo-ocular reflex (VOR) stabilizes images of stationary objects by generating eye movements opposite in direction and nearly equal in amplitude to head movement. Smooth pursuit stabilizes images of small moving objects by generating slow tracking eye movements. Much of our work concerns motor learning in the VOR. Since the oculomotor system provides model in which rigorous techniques can be used to study brain function in behaving monkeys, the work will be relevant to the problem of motor learning in general. Thus, the results may have implications for people who must learn new ways to make old movements, such as individuals who have had strokes, or children born with developmental disorders. In addition, smooth eye movements involve structures near the junction of the brainstem and cerebellum (pontine angle); a complete understanding of the mechanisms of smooth eye movement would facilitate early diagnosis and treatment of tumors and diseases that commonly affect this brain region. Experiments are done in rhesus monkeys that receive liquid rewards for tracking a small, moveable target. 1) Psychophysical experiments will measure the response of the vertical pursuit system to a variety of precisely controlled retinal stimuli. The aim of this project is to determine what retinal inputs can affect pursuit and what inputs are actually used in normal tracking. 2) Single cell recordings will monitor the discharge of cells throughout the VOR pathways during eye movement and vestibular stimulation. Each cell's afferent and efferent connections will be studied using a) rapid changes in head velocity as a vestibular stimulus and b) post-spike averaging in the abducens nucleus. The aim of this project is to determine the connections of cells whose discharge suggests they participate in the VOR. 3) Neurophysiological experiments will investigate the site of neural changes underlying long-term visual modification of the VOR. Rapid changes in head velocity will estimate a cell's sensitivity to vestibular inputs. Extraocular motoneurons will be studied first, to identify in adapted monkeys the nature and magnitude of changes we are looking for. Other brain stem cells will then be studied to identify those that provide the changed inputs.